Re: [PATCH] mm: check the return value of lookup_page_ext for all call sites

Minchan Kim
2016-05-30T06:10:46Z

On Fri, May 27, 2016 at 11:16:41AM -0700, Shi, Yang wrote:

<snip>

> >
> >If we goes this way, how to guarantee this race?
>
> Thanks for pointing out this. It sounds reasonable. However, this
> should be only possible to happen on 32 bit since just 32 bit
> version page_is_idle() calls lookup_page_ext(), it doesn't do it on
> 64 bit.
>
> And, such race condition should exist regardless of whether DEBUG_VM
> is enabled or not, right?
>
> rcu might be good enough to protect it.
>
> A quick fix may look like:
>
> diff --git a/include/linux/page_idle.h b/include/linux/page_idle.h
> index 8f5d4ad..bf0cd6a 100644
> --- a/include/linux/page_idle.h
> +++ b/include/linux/page_idle.h
> @@ -77,8 +77,12 @@ static inline bool
> test_and_clear_page_young(struct page *page)
> static inline bool page_is_idle(struct page *page)
> {
> struct page_ext *page_ext;
> +
> + rcu_read_lock();
> page_ext = lookup_page_ext(page);
> + rcu_read_unlock();
> +
> if (unlikely(!page_ext))
> return false;
>
> diff --git a/mm/page_ext.c b/mm/page_ext.c
> index 56b160f..94927c9 100644
> --- a/mm/page_ext.c
> +++ b/mm/page_ext.c
> @@ -183,7 +183,6 @@ struct page_ext *lookup_page_ext(struct page *page)
> {
> unsigned long pfn = page_to_pfn(page);
> struct mem_section *section = __pfn_to_section(pfn);
> -#if defined(CONFIG_DEBUG_VM) || defined(CONFIG_PAGE_POISONING)
> /*
> * The sanity checks the page allocator does upon freeing a
> * page can reach here before the page_ext arrays are
> @@ -195,7 +194,7 @@ struct page_ext *lookup_page_ext(struct page *page)
> */
> if (!section->page_ext)
> return NULL;
> -#endif
> +
> return section->page_ext + pfn;
> }
>
> @@ -279,7 +278,8 @@ static void __free_page_ext(unsigned long pfn)
> return;
> base = ms->page_ext + pfn;
> free_page_ext(base);
> - ms->page_ext = NULL;
> + rcu_assign_pointer(ms->page_ext, NULL);
> + synchronize_rcu();

How does it fix the problem?
I cannot understand your point.

> }
>
> static int __meminit online_page_ext(unsigned long start_pfn,
>
> Thanks,
> Yang
>
> >
> > kpageflags_read
> > stable_page_flags
> > page_is_idle
> > lookup_page_ext
> > section = __pfn_to_section(pfn)
> >offline_pages
> >memory_notify(MEM_OFFLINE)
> > offline_page_ext
> > ms->page_ext = NULL
> > section->page_ext + pfn
> >
> >>
> >>Thanks.
> >>
>